Boron nitride (BN) is a well-documented, non-oxide ceramic whose synthesis can be achieved by many routes. The traditional routes are described in "Gmelin Handbuch der Anorganischen Chemie"; Springer-Verlag: West Berlin, 1988: 3rd Supplement, Vol. 3, and references therein. Many of the traditional routes to boron nitride are tedious, multi-stepped, costly, and do not provide routes for producing thin films of BN. One route to BN, and thin films thereof, involves chemical vapor deposition (CVD). Unfortunately, CVD techniques require special equipment and complex preparation.
Molecular precursors of BN are now being pursued as possible synthetic routes for thin films of BN. Recent interest in finding molecular precursors of BN by Narula et al. (Chem. Rev. 1990, 90, 73-91) has been spawned by the need for soluble and/or fusible pre-ceramic polymers in some advanced ceramic applications. An appropriate candidate for a BN molecular precursor must have certain characteristics and requirements including simple chemistry and non-elaborate techniques to obtain BN from the precursor.
Ideal molecular precursors to BN should already have the appropriate stoichiometry of boron and nitrogen, and require nothing more than thermal activation to achieve BN. Ammonia-borane, H.sub.3 NBH.sub.3, would be the ideal candidate since it has the desired solubility in organic solvents and the appropriate ratio of boron and nitrogen is already present. The decomposition of H.sub.3 NBH.sub.3 has been studied both in the solid state and solution. (Wang and Geanangel, Inorg. Chim. Acta. 1988, 148, 185-190; Komm et al., Inorg. Chem., 1983, 22, 1684-1686) However, one major drawback is the high volatility of H.sub.3 NBH.sub.3 under mild thermal conditions. Ammonia-borane sublimes at temperatures near 112.degree. C. and as a result the yield of BN is depleted. (Walker et al., Ceram. Bull., 1983, 62, 916-923). In addition, H.sub.3 NBH.sub.3 decomposition produces volatile materials, i.e., B.sub.2 H.sub.6 and B.sub.3 H.sub.3 N.sub.3 H.sub.3, which lowers the yield of the BN desired by removing the BN from the source. In order to overcome this, a reactive site must be present in the pre-ceramic unit so that upon thermolysis the maximum yield of BN can be attained.
Sneddon (Chem. Mat., 1989, 1, pp. 443-448) has demonstrated a synthesis of BN from the reaction of commercially available dimethyl sulfide dibromoborane, (CH.sub.3).sub.2 SBHBr.sub.2 and ammonia gas to form the unstable dibromo intermediate H.sub.3 NBHBr.sub.2. This unstable intermediate can be used to create a solution for coating articles or substrates followed by exposure of the coating to ammonia gas and heat sufficient to cause the evolution of dimethyl sulfide, hydrogen, and hydrogen bromide to form BN. However, this method of Sneddon requires the use of ammonia gas, unlike the present invention, and relies upon the unstable dibromo intermediate H.sub.3 NBHBr.sub.2.
Therefore a need exists for a H.sub.3 NBH.sub.3 -like material useful as a precursor for the thermal preparation of BN.